2002 Mercedes Benz A160 SRS light

the car I have is a A190 2002. The problem is the srs light keeps coming on, I have had it checked at my local mecedes dealership and they can not find the fault, is this a common problem? The mechanic thinks it is abroken wire somewhere, but at a$150per hour I have had to say leave it as they have already spent 3 hours triying to locate the problem